The cutter units of a rotary cutterbar operate very close to the ground and include discs that rotate at a high rate of speed so that the knives carried by the discs are able to impact-cut the crop. This combination of low location and high speed results in a high wear rate for the knives. In addition, the knives are also damaged by contact with rocks and other obstacles. It is accepted practice to replace the knives on a regular basis due to wear. So that the nut of the knife mounting bolt does not become damaged, it is known to provide a shield forming a cup or well which surrounds the nut. U.S. Pat. No. 5,845,468, issued on Dec. 8, 1998 to Richardson et al. discloses such a shield. This prior art shield has the drawbacks of being relatively complicated and requiring the disc to be specially designed with a notch to receive the shield. Further, the well of the shield which surrounds the head of the mounting bolt has a tendency to collect crop material which must be cleaned out to permit a socket to engage the nut of the knife mounting bolt whenever it is desired to replace the knife. Also, it is known to equip cutter discs with crop guide vanes or paddles located in the vicinity of the shield which necessitates that the vanes be designed for providing clearance for the shield, an example of such vanes being disclosed in U.S. Pat. No. 5,842,330, issued on Dec. 1, 1998 to Richardson et al.